Is it necessary to define a positive deviation for the wall thickness tolerance of heat exchange tubes?
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For heat exchange tubes, refer to GB/T150; for tube tolerances, refer to GB/T9948 and GB/T151; for tube tolerances, refer to NB/T47019.1. As shown above, taking a tube with dimensions of φ19×2 as an example, the allowable outer diameter tolerance is ±0.1 (GB/T151) or ±0.08 (NB/T47019.1), while the allowable wall thickness tolerance is ±0.2 (GB/T9948) or ±0.15 (NB/T47019.1). But when I searched online, someone mentioned that the tolerance for the wall thickness of pipes should be a positive deviation, giving +0.2/0 as an example. Is this reasonable? Is there no standard basis either? Won’t there be any difficulties when purchasing from the market?
There are no specific design specifications; generally, national standards are followed. Even for the thickness of a single pipe, there can be both positive and negative values. Requiring all values to be within the positive range means that in some cases the thickness will exceed the specified limit. So why not choose steel pipes of a higher thickness grade? What you mentioned, 19+0.2/0, corresponds to 19.1±0.1. Requiring all values to be positive is, in effect, asking the steel manufacturer to improve the precision of production. Unless the manufacturer produces the pipes specifically for you, it’s not possible to find such pipes on the market
All requirements specify a positive deviation; this is basically the standard for heat exchange tubes in foreign-funded projects. It just needs to be specified in the purchase contract. Steel pipe factories can do it. I have worked on many such projects. 1: Increase service life ； 2: Welding the pipe ends is easier ；
It has become standard practice for the bidding requirements set by various design institutes and engineering companies to specify positive-pressure pipes, so there are no issues with procurement.
Following the standards means overly strict requirements, so no one takes on the orders.